Loud-speaker horn



April 21, 1931- J. w. MILNOR 1,801,521

LOUD SPEAKER HORN Filed July 18, 1927 INVEN TOR.

JZ1/177Zz77207;

a wmw ATTORNEY Patented Apr. 21, 1931 JOSEPH MILNOR, OF IAPLEWOOD, NEWJERSEY LOUD-SPEAKER HORN Application filed July 18,

M invention relates generally to loud spea ers of the horn type, used inphonoraphs', radio receiving sets, and public adress s stems forchanging electrical energy or mec anical motion into energy of soundcapable of reaching a number of listeners.

One of the objects of my invention is to provide a loud speaker whichwill be practically equally effective in ereating or reproducing soundwaves of all frequencies which are of importance, and will thus cause aminimum of distortion of the sound energy.

Another object is to provide a horn loud speaker which will beespecially efficient inarnplifying tones of low frequency.

A further object is to provide a horn loud speaker which will occupyless space than was previous considered necessary in order it to obtaingoo results.

the physical dimensions of the horn; however such a horn may be unwieldyand expensive. As is well'known, audible sound is composed ofoscillations rangin from about 16 vibrations or cycles er secon toupwards of 15,000 per second. atisfactory reproduction of music and thevoice requires that av range of frequencies at least from 100 to 5000cycles shall be handled with nearly equal efficiency, and somewhat morenatural reproduction will be obtained if this range is increased tocover. frequencies from 50 to 7000 cycles.

In the present invention the effectiveness of the horn at the lowerfrequencies is improved by decreasing the velocity of the sound withinthe horn, which is accomplished by increasing the effective density ofthe medium within the horn. The decrease in velocity might, for'example,be obtained by using in the horn, instead of air, a gas suc as sulphurdioxide, which is materlally heavier than air. I have found that thedecreased 1927. Serial No. 206,632.

velocity can be obtained more satisfactorily by placing a series of thinsheets of flexible material, such as paper, throughout the length of thehorn, with their planes perpendicular to the axis of the horn. A hornconstructed in this manner may be said to be loaded. The sheets must beheld in position in "such a manner that they may move freely with thevibrations of the air, and they in effect increase the weight of themedium which carries the sound. The sheets are so supported that theyare free to vibrate in a direction perpendicular to their planes inresponse to thewaves of sound. In some cases the necessary freedom ofmotion may be obtained from' the inherent flexibility of the thinmaterial. If this is not suflicient, some supporting means may beemployed at the margins to hold them in relative position. If thismethod is properly applied it will not affect the reproduction at thehigher frequencies, but will materially improve operation at the lowerfrequencies, and willextend the range downward.

My invention will be more clearly understood by reference to theaccompanying drawin s, in which:

igure 1 is a central longitudinal sectional View of a horn whichembodies my invention. It is understood that a cross-section along aline such as AA might show the horn to be circular, or square, or ofother shape asdesired. The line B-B represents the axis of the horn. l

Figure 2 is a front'elevation of the horn shown in Figure 1.

Figure 3 is an alternative construction for the front end of the horn.

Figure 4 is a front elevation of another alternative construction forthe end of the horn.

Figure 5 is a sectional view on the line 55 of Figure 4.

Figure 6 shows an driving the horn.

Figure 7 is a central longitudinal sectional view of a horn embodying myinvention in which the sound-transmitting medium consists of a gasheavier than air which is conalternative method for fined within thehorn and completely fills the same. 7

In Fi re 1 I have shown the shell 8 of the horn w ich confines anddirects the waves of sound formed with the axis a straight linebut it,will be understood that the axis of the horn may be' bent or the hornmay be folded back in order to conserve space without invalidating theprinciples involved in this invention. I have indicated diagrammaticallyan electromagnetic driving element to actuate the diaphragm 9, which mayconsist of a permanent magnet 10 having an armature 12 pivotally mountedthereon and adapted to oscillate under the influence of a coil orwinding 13,. in accordance with electric current impulses from a radioreceiving circuit, or

from a telephone transmitter or an amplifier.

The increase in the eifective densit of the sound transmitting mediumwithin t e horn may be accomplished by supporting at their edges withinthe horn a number of thin sheets 14 of flexible material, with theirplanes perpendicular to the central axis of the horn and preferablyspaced at equal distances.

order to decrease reflections at the large I opening or hell of thehorn, a bafile may be mounted therein having a largenumber of openings16. Thisbaflle is fairly rigid and may consist of a number of slats 15of wood, or other suitable material. It is "preferable that the sides ofthe slats should taper outwardly as indicated in' Figs. 1 and 2, butthis is not essential and they may be made as shown in Fig. 3. Anotherform of bafiie is shown in Figs. 4 and 5, which consists of a late 17,erforated with a large number of oles 18 t at preferably taper inwardlytoward the horn.

The diaphragm 9 may be vibrated by a mechanically actuated drivingelement such, for instance, as I have indicated in Fig. 6. 'A lever 19pivoted at 20 and carrying a needle 21 is vibrated by a rotatingphonographic record 22 and causes corresponding vibrations of thediaphragm.

In Figure 7 I matically a form of my invention in which the outer end ofthe horn is closed b a diaphragm 23-and in which the horn 1s filled witha gas denser than air to serve as a soundtransmitting medium instead ofthe thin sheets 14 in the form illustrated in Figure 1. The diaphragmis, of course, constructed of a thin airht flexible material which is soconstructe and supported that it is free to vibrate with the sound waveswithout retardmg them.

he theory of this invention will be de-- scribed in connection with theso-called exponentialftype of horn, although in slightly modified formitmaybe used with other designs of born. In the exponential horn, the

area at a section AA distant 4v centimeters from the throat is given bythe equation:

have illustrated diagram A,- =A.,B where A. is the area of the throat, Bi; a-lconstant depending on the design, and .7 8.

In an article written by C. R. Hanna and J. Slepian, published in theTransactions of the Institute of Electrical Engineers of February, 1924,it is shown that the cut-off freuency of a horn is approximately given.by

t e formula: 1 B B 1 a 0 I LETHYM where a is given as the velocity ofsound in air. Frequencies below the value given are radiated weakly ornot at all. Frequencies just above this value are radiated weakly,

and frequencies materially above this value are radiated efiiciently andwith equal force. It is here assumed that the large end of the horn isproperly designed; this feature will be further referred to later. i Ifthe air within the horn is replaced -b a medium other tha air, thetheory shows that the cut-off frequency is given by the formula MB L j.-

where 0.1 is the velocity of sound in the me dium. It therefore followsthat the cut-01f frequency may be lowered by any means which decreasesthe velocity of sound within the horn.

A well known formula for the sound in a medium is,:-

velocity of in which 1) is the absolute pressure in -dynes/cmF; p is themass of medium in grams percmfi; and 7c is the ratio of the two specificheats of the medium (1.41 for air). Therefore by increasing the mass ofthe medium, the cut-off frequency may be lowered. A I

In a wave travelling alon the axis of the horn the particles of themedium oscillate in a direction approximately parallel to the axis, andit is necessaryonly to increase the mass that is moved in thatdirection. I accomplished this purpose by suspending a suflicient numberof sheets of thin material of proper weight'in the manner shown. Thesheets must be suspended in a manner such that they are free to move. 7In formula (3),

p is then the sum of the mass of the gas,

plus the average mass of the sheets of material, per cubic centimeter ofthe space within the horn. I

If the loud speaker were required to reproduce only low frequencies, "acomparatively small number-of sheets of heavy material would besuflicient. Such an arrangement, however, would interfere withreproduction of higher frequencies. In order to operate satisfactorily,have found that they y as previously described an it is desired to reprouce, the wave length being measured along the axis of the horn.

The wave length of the highest frequency f is given by and the distancebetween sheets should be not greater than about shell of the horn (i.e., thevalue of B, above,

should vary at different points alon the length of the horn, with theamount of oading varied correspondingl or perhaps omitted in a part ofthe horn). If the large end of the horn were to open directly into theair, there would be more or less reflection back from the open end ofthe horn, which would tend to accent certain frequencies, and suppressothers. Such reflections would be caused by the condition that the ratioof velocity to pressure of the particles of gas in the sound within thehorn 15 not the same as the corres onding ratio in air outside of thehorn. This effect ma be reduced or avoided by groviding a bafllz theproportions ifs which may be computed as follows 2- A,=area of large endof horn (just before reaching baflle).

A =combined area of all spaces between bafl le plates (at point wherearea is smallest).

: P, =pressure (above atmospheric pressure) at a sound wave in the hornjust before reaching the baflle (dyneS/cmF). i

V =.velocity of particles of gas in a wave at the same point (cm./sec.).P and V =pressure and velocity in the wave between elements of the baflie. g=density of air.. o.=velo city of sound in air. Then from the wellknown theory:

If theentire ener in the-wave atA is to pass out between aflle plates,

. 1 l l l b b h and Tit-Ia Combining, this gives:

which re resents approximately the best area of t e combined spacesbetween the baflles.

After passin between the baflles, the sound issues as a p lurality ofseparate waves. These combine a short distance beyond the baflle to forma single wave of sound. v

The above theory of the baflle is most nearly correct at the higherfrequencies. At lower frequencies. there is a tendency for the pressureat the baffle to be a little less than given by Equation 7 due to theamount that the wave s reads (within the first half wave length) a terleaving the bafiie. This effect may be partially compensated for byusing as the best area of the spaces between baflie lates, a value slihtly less than that given y Equation 9Q hus even in a horn without theloading provided by the sheets of flexible material, the use of a bafilewill somewhat strengthen the lower frequencies.

The distance between adjacent s aces (1616, or 18-18, in the figures ofthe rawing) must be small as compared with the wave length in air of thehighest frequency it is desired toreproduce with the horn. Thisis'important, as otherwise the higher frequencies would have to traveldifferent distances to reach an o ning in the baflle, which would causeinter frence which would result in more or less loss of the energy inthese frequencies.

Figures 1 and 5, the area of the dia- P g of the throat 7 It should benoted, however, thatin practice the diaphragm may be made either largerthan, equal to, or smaller than the throat. The best'ratio of the areasde "phragm to be caused to operate by sound waves in air.

I cla1m:

9 is shown as larger than the area 1.- In a loudspeaker of the horntype, means distributeduniformly throughout the horn for decreasing thevelocity of the sound waves I for decreasing the v in their passagethrough the horn as compared with their normal velocity in air saidmeans comprising for useful frequencies a substantially non-absorbingsound transmitting medium having a mean density greaterthan that of air.

2. A loud speaker, embodying a horn and means distributed uniformlythroughout the horn for increasing the effective density of thesound-transmitting medium therein, said means consisting of asubstantially non-ab sorbing sound transmitting medium uniformlydistributed therein having a mean density greater than that of air.

3. A loud speaker, comprising a horn having a plurality of sheets ofthin non-porous material disposed therein at substantially equaldistances from each other along the axis of the horn, each of whichsheets occupies substantially the entire cross-sectional area of thehorn.

4. 'A horn loud speaker, embodying a plurality of sheets of thinmaterial mounted therein with their planes approximately perpendi'cularto the axis of the horn, the sheets being spaced at distances notgreater than about one-third of the wave length of the highest frequencyit is desired to reproduce.

5. In a horn loud speaker, a bafile at the large end having a pluralityof openings, the

openings being separated by distances smaller than thewave length of thehighest frequency it is desired to reproduce.

6. In a horn loudspeaker, a battle at the large end comprising aplurality of baflie plates, the approximate combined area of spacesbetween baffle plates being given by the formula:

7.111 a horn loud speaker, a baflle a the large end comprising aplurality of,-baflie lates, the combined area of spaces between afiieplates being not greater than as given bytheformulaz, a

-8. In a horn loud slpeaker, means therein e ocity of sound wavestransmitted-therethrough comprising for useful frequenciesasubstantially non-absorbing sound transmitting medium distributed withinthe horn and having a mean density greatthe comhorn,'and a baflle at thelarge end having a plurality of openings, the combined area of which ismaterially less than the area of said on 10. In a loud speaker, providedwith a baflle having a plurality of openings, said openings tapering sothat their larger area is at their outer ends.

11. In a horn loud speaker, a baflie at the large end, the baflie havinga plurality of openings, the combined area of which is materially lessthan the area of said end, and the openings being separated by distancessmaller than the wave length of the highest frequency it is desired totransmit.

12. In an organization for reproducing or transmittin sound, a rigidbaflie at. the outer end, the ba e having a plurality of openings thecombined area of which is materially less than the area of said end, andthe openings being separated by distances smaller than the wave lengthof the highest frequency it is desired to transmit.

13. In an orga'nizationjor reproducing or transmitting sound; a rigidbafile at the outer end, the bafliehaving a plurality of openings, thecombined area of which is materially less than the area of said'end, andthe openings being separated by distances smaller than the wave lengthof the highest frequency it is desired to transmit, said openingstapering so that their larger area isat their outer ends.

14. In an organization for reproducing or I transmitting sound, a hornclosed by diaphragms at its ends, means therein for decreasing thevelocity of sound waves transmitted therethrough'consisting of a gasheavier than air. v s a In testimony whereof I 'aflix my'signature.

. JOSEPH W. MILNOR.

,er than that of air, and a bafiie at the large end having a pluralityofopenirligs, bmed area of vvhich is materia y less than v the ar a ofsaid end.

9. a horn loud speaker, a plurality of l sheets of thin material held inposition therein with their planes approximately perpen dicularto thelongitudinal axis of the horn,

each of which sheetsextends over substantially the entirecross-sectional area of the

